The math behind ND filter exposure conversion is genuinely simple once you've seen it worked through once — here's the method step by step, with two full examples.
The formula
new shutter speed = base shutter speed × 2stops
Each stop of ND halves the light reaching the sensor, so to hold exposure constant, shutter speed must double for every stop added. This is the exact formula this calculator's exposure field runs.
The three steps
- Meter your base exposure without the ND filter attached. Get your correct shutter speed for the scene at your chosen aperture and ISO using your camera's own meter — ideally before screwing on a strong filter, since metering through a very dark ND can be unreliable (see the metering page).
- Identify your ND filter's stop value. Use the stop chart if you only know the ND number (ND8, ND1000, etc.).
- Multiply base shutter speed by 2 raised to that stop count. That's the new shutter speed to set in bulb mode (or a timed exposure, if your camera supports a speed that long directly).
Worked example 1: a waterfall at 1/15s with a 6-stop ND
Base shutter speed 1/15s = 0.0667s, ND64 = 6 stops. 0.0667 × 2⁶ = 0.0667 × 64 = 4.27 seconds — a classic silky-water exposure time, achieved without touching aperture or ISO.
Worked example 2: a cloudscape at 1/60s with a 10-stop ND
Base shutter speed 1/60s = 0.01667s, ND1000 = 10 stops. 0.01667 × 2¹⁰ = 0.01667 × 1024 ≈ 17.07 seconds — long enough to streak moving clouds while keeping static elements (buildings, rock formations) fully sharp.
Why the result jumps so fast per stop
Because the multiplier is exponential in stops, not linear — each additional stop doesn't just add a fixed amount of time, it doubles whatever you already had. That's why a 10-stop filter (1024× longer) produces such a dramatically longer exposure than an 8-stop filter (256×) even though they're only 2 stops apart — the difference between them is a full 4× multiplier on top of whatever base time you started with.
Checking your own math against the calculator
Enter your own base shutter speed and ND strength into the calculator directly — it runs the identical formula shown above and additionally formats the result into human-readable minutes/seconds, which becomes genuinely useful once exposures run past about a minute.
FAQ
- Does this formula change for mirrorless vs. DSLR cameras?
- No — the physics of light transmission through an ND filter is identical regardless of camera type; this is a pure optics/exposure calculation independent of sensor or viewfinder technology.
- What if my base shutter speed isn't a "clean" fraction?
- The formula works with any decimal value — enter your custom base shutter speed in seconds (e.g., 0.004 for 1/250s) in the calculator's custom field rather than needing a preset to match exactly.
- Does aperture or ISO factor into this calculation at all?
- No — this formula assumes aperture and ISO stay fixed between the base (no-filter) and final (filtered) exposure; it only converts the shutter-speed side of the exposure triangle.
→ Calculate an exposure (free)